Abstract
Purpose: :
Previous studies with the Oculus Pentacam suggest that an Equivalent K Reading (EKR) zone of 4.5 mm is best for estimating corneal power in postrefractive patients (Holladay et al., In review JCRS). Other studies suggest a smaller EKR zone of 2-3 mm is optimal for predicting corneal power (Hill et al. 2008 CRST). The purpose of this study was to determine the best EKR zone for estimating IOL power in postrefractive patients.
Methods: :
Retrospective review of 10 eyes from 8 patients ages 47 to 69 who underwent cataract surgery after prior corneal refractive surgery (myopic LASIK, PRK or RK). For each case, the residual refractive error was calculated as the difference between the actual postoperative refractive error and the preoperative target refractive error. This information was then used to calculate the ideal predicted IOL power based on a linear regression model (Jin et al., 2007 Cornea). IOL power calculations were then performed with preoperative Pentacam keratometric data from each of 5 EKR zones (1mm, 2mm, 3mm, 4mm and 4.5mm). These IOL calculations were then compared to the ideal IOL power determined from the regression, and a best EKR zone was determined retrospectively.
Results: :
As summarized in Table 1, postoperative refractive error ranged from -1.375 D to +2.125 D for the 10 cases reviewed. The average magnitude of refractive error was 1.175 D. For 9 of 10 cases, best-corrected visual acuity was 20/25 or better. The average best EKR zone for the group was 2.2 mm. In 4 of 10 cases, a 1mm EKR zone was best. For the remaining 6 cases, EKR zones of 2mm, 3mm, and 4mm were best. There were no cases in which an EKR zone of 4.5 mm was the best .
Conclusions: :
The Oculus Pentacam provides useful keratometric data for calculating IOL powere in patients who have previously had refractive surgery. Care should be taken in interpreting this data and selecting an appropriate EKR zone for IOL calculations. Our results suggest that 4.5mm is not the best optical zone for estimating IOL power, and smaller zones (1-4mm) should be used. Further studies are needed, however, to identify other factors that may assist in selecting the optimal EKR zone for each case.
Keywords: cornea: clinical science • cataract • refractive surgery